1. Field of the Invention
The present invention relates to an image processing apparatus, such as a color copying machine, color printer, and color facsimile, which applies color correction to separated color signals in the input section and converts them into recording signals used for applying a recording color material, and outputs a color image.
2. Background of the Invention
Referring to FIG. 33, there is shown an arrangement of a color balance/color correction circuit in an image processing apparatus, which is disclosed in Japanese Patent Application No. 63-273857, filed by the applicant of the present patent application.
In FIG. 33, an IIT (image input terminal) 700 contains a CCD line sensor with color filters, for example, and reads a color original in terms of separated color signals of B (blue), G (green), and R (red). An END converting circuit 701 corrects variations in the spectrum of an illumination light source, characteristics of a dichroic mirror, and color characteristics of photo-electric converting elements, color filters, lenses, and the like. The same further applies correction to the negative to positive conversion when a filter projector is used, and difference of film transmittances that are different for different film makers. After such corrections, the END converting circuit produces gray-balanced, separated color signals ENDB, ENDG, and ENDR. A color masking circuit 702 operates matrices involving the gray balance process, such as 3.times.3, 3.times.6, and 3.times.9 matrices, into which the gray balance method is applied in accordance with ratios of the gray-balanced, separated color signals ENDB, ENDG, and ENDR as output from the END converting circuit 701. In a full color mode, the color masking circuit generates gray-balanced, pixel (picture element) data of three color toner signals ENDY. ENDM, and ENDC. In a mono color mode, it generates a luminescent signal. A UCR (under color) circuit 703 generates a color signal of K depending on ratios of the toner signals ENDY, ENDM, and ENDC in a 4-color full color mode, and applies the subtraction to the toner signals ENDY, ENDM, and ENDC in accordance with a value of K. In the process of generating the K, a maximum/minimum detector 7031 detects maximum and minimum values of the toner signals ENDY, ENDM, and ENDC. A subtractor 7032 works out a difference between the maximum and minimum value of each toner signal. A value as converted in a chroma function 7033 in accordance with each difference, is subtracted from the minimum value in another subtractor 7034. In the under color process, a value as covered from the K in an UCR function 7034 is subtracted from the values of the toner signals ENDY, ENDM, and ENDC respectively in subtractors 7036Y, 7036M, 7036C. In 3-color full color mode and the mono color mode, the generation of K and the under color removal of ENDY, ENDM, and ENDC are not carried out. The UCR circuit 703 is by-passed. A TRC (tone reproduction control) determines values of output pixel data for the input pixel data on the basis of TRC curves. An IOT (image output terminal) 705 reproduces the color original in the form of a mesh point as described by tone adjusted tone signals.
As described above, a color image process apparatus is capable of generating a mono color image or monochrome by generating a luminous signal on the basis of the separated color signals of B, G and R, and by using an image of single toner or superposed images of two different toners. So far as we know, there is no prior art clearly describing how to process the image data or what toner is used for forming an output image in the mono color mode.
When the parameters as used in the full color mode are directly applied to the TRC circuit, there occurs no problem in particular in reproducing an image, such as a graph. In a half tone image, such as photograph and printing, a shadow part (high density or dark part) in an original is defaced. Accordingly, the resultant image may not have good quality. In other words, when the normal parameters are used, the gamma characteristic is overemphasized. The contrast is poor, and a gradation of the if tone image is unnatural, in which gradation indicates the degree of density of black/white and intermediate colors in terms of steps of visual brightness. Then, an image reproduced lacks natural feeling.
Recently, computer graphics, for example, have frequently employed a method wherein much visual information is presented in an area painted out by a half tone, and other visual information such as images and characters are additionally displayed in the painted out area. In this case, poor gradation properties (indicating degree of the step of gradation) of the half tone provide illegible presentation of the visual information in the half tone or painted out area. Consequently, advantages arising from the color display cannot fully be utilized.